This invention relates to helical traveling wave tube amplifiers, useful in amplifying RF signals in communications, data transmission, broadcasting, satellite and radar mapping applications. A novel geometry and sever eliminates destructive interference within the amplifier, and results in significantly improved efficiency.
A traveling wave tube (TWT) is a device used to amplify an RF signal in a high vacuum environment. The RF signal is amplified by the interaction of the RF wave with a beam of electrons at high voltage. The electrons are emitted from an electron gun, a thermionic emitter of electrons, using a heater to achieve required temperatures, up to 1000xc2x0 C. or more. The RF signal is typically in the range of 500 MHz to 50 GHz. A traveling wave tube used to accomplish this amplification may be of either the close-coupled cavity type or the helical type. The helical type has been favored because of its simpler construction, lower cost and large band width. Both types of amplifier, however, suffer low electronic efficiency. Other disadvantages follow from high skin effect losses, resulting in part from high helix temperatures. This typically translates into a need for greater heat transfer. High temperatures also create higher I2R losses in the helix itself, due to the simple fact that electrical resistance increases with temperature. These disadvantages also limit the performance of the amplifier, and require more amplifiers be used if fewer amplifiers cannot meet the needs of the application.
The need for improvement in helical tubes has been recognized and many suggestions have been made over the years. Instead of ordinary helical sections, shaped conical sections have been proposed. Varying and reducing the pitch between repeating elements of the helix have been suggested. One improvement by the inventor of the present invention, U.S. Pat. No. 4,564,787, and incorporated here by reference, involved a dynamic velocity taper, varying the pitch of the helix at an exponential rate, while keeping the helix radius constant.
When the gain of an amplifier exceeds about 20 dB, there may be backward wave oscillations of the RF signals input into the amplifier. These backward oscillations render the amplifier less useful. In addition to these undesirable oscillations, unacceptable fluctuations in amplifier gain and phase may be caused by inaccuracies in the construction of the amplifier. Fluctuations may also be caused by reflections from mismatches in the output load or terminations. One way to improve the efficiency of traveling wave tubes is to use a sever. A sever is a conductive path between the helical conductor of a traveling wave tube and the housing which is typically made of metal. A sever functions as a form of filter, helping prevent such backward oscillations and helping to smooth out fluctuations in the amplifier gain. The sever is intended to reduce backward waves on the helix without interfering with the xe2x80x9cbunchingxe2x80x9d or slowing of electrons in the electron beam. However, even a good conventional sever may reduce the bunching, represented by the amplitude of RF current in the beam, by about 15%. This contributes directly to a loss of efficiency in the amplifier.
A sever in a traveling wave tube consists of a short region, typically 1 cm. to about 5 cm. long, depending on the frequency of operation. The sever consists of a conductive material deposited on the dielectric rods used to support a helical conductor in a housing. When the gain of the amplifier approaches about 20-25 dB, a sever is placed at an input end of the helical conductor to produce a gradual, exponentially increasing electrical contact. The sever reduces signals by about 50-60 dB in the middle of the sever. Then the sever begins an exponentially decreasing contact, down to a level of no attenuation at the opposite end of the sever, on the side toward the output end of the helical conductor.
Severs have two disadvantages. They reduce electronic efficiency by about 2%, which translates into an even greater loss in overall efficiency in the amplifier as a whole. They also add approximately 2 to 5 cm in length to the helical conductor, as the amplifier is lengthened to make up for the electrical loss. Overall, such severs add about 20% to the length and weight of each amplifier, and can reduce efficiency by approximately 20%.
What is needed is a construction that will prevent backward oscillations of RF waves and prevent fluctuations in the amplifier gain. What is needed is a sever that can prevent backward oscillations without debunching of electrons and loss of this efficiency in the amplifier. What is needed is a sever without the undesirable properties of present severs.
A key to increasing efficiency in the traveling wave tube amplifier by means of a sever is to limit the attenuation of the sever. Rather than providing a high degree of attenuation, it has been found that an increase in efficiency may be attained by a lower degree of attenuation.
One aspect of the invention is a helical traveling wave tube amplifier for amplifying an RF signal. The amplifier includes a traveling wave tube with an anode, a cathode and a collector. A negative potential is applied to the cathode, inducing a beam of electrons to the anode, which is typically at ground potential. The beam passes through to the collector. The beam of electrons amplifies an RF signal input to a helical conductor. The helical conductor includes an RF input, an input section, a middle section, an output section, and an RF output. The input section of the helical conductor includes an adverse space harmonics taper.
The amplifier is constructed so that helical conductor is between the cathode and the collector, and the beam of electrons travels through the center of the helical conductor. The traveling tube amplifier and the helical conductor are enclosed in a housing, and are operated at high vacuum. At least one dielectric rod provides support for the helical conductor inside the housing. A sever is constructed between the helical conductor and the housing, by providing a conductive coating on the dielectric rods. The sever desirably provides about 1 to about 30 dB of attenuation, as portions of the RF signal, and desirably any backward oscillations, are bled off.
Another key to the invention is to recognize the importance of the interaction between the electron beam and the RF signal. The reason that traveling wave tube amplifiers are sometimes called xe2x80x9cslow wave structuresxe2x80x9d is that the RF signal is traveling much faster than the generated electron beam, and the RF signal must be slowed down for interaction with, and amplification by, the electron beam. The formation of a helical path is the first step in the slowing process and is recognized as a means of lengthening the path. In one embodiment of the invention, the sever is used with a conductor having a helical path of varying radius, in conjunction with a simultaneously varying pitch, forming an adverse space harmonics taper (ASHT) in part of the helix. Adverse space harmonic tapers have been described in co-pending U.S. pat. application Ser. No. 09/612,035, assigned to the assignee of the present application. It has been discovered that such a structure is capable of achieving far greater interaction between the RF signal and the electron beam, and thus achieving greater electronic efficiency in the amplification, and greater efficiency overall in the performance of a traveling wave tube. The use of the ASHT and the improved sever lead to an overall increase in efficiency of about 10% or more.
One embodiment of the invention is a helical traveling wave tube, which includes a helical conductor with an RF input and an RF output, and an electron gun positioned concentrically with respect to the helical conductor. The electron gun consists of a negatively-biased cathode and a grounded anode, both at a near end of the helical conductor. There may also be a control grid downstream of the anode, still at the near end, and a collector at the far end of the helical conductor. The electron gun may be run in a DC mode or may be pulsed as desired through the cathode or the grid. A series of magnets surrounds the outside of the helical tube, for a magnetic field to focus the beam of electrons passing from the cathode to the collector. At least the portion of the apparatus comprising the electron gun, the helical conductor, and the RF input and output should be operated in a hard vacuum.
The helical conductor has an input section corresponding to an RF input and an output section corresponding to an RF output. In a preferred embodiment, a portion of the helical conductor contacts a portion of the support structure between the helical conductor and the housing. The support structure includes at least one dielectric rod between the helical conductor and the housing, and a sever is located near at least one point of contact. The sever desirably provides about 1 to about 30 dB of attenuation of an RF signal input to the helical conductor, and eliminates backward oscillations of the RF waves.